On modern rock-drilling rigs, the positions of the drill booms are measured in order to be able to drill the holes in the correct place in the rock, possibly by using automatic controls. When measuring a position for a drill hole, the angles are measured at each joint of the drill booms and any telescopic movement of these. A boom has normally five to six joints, for which reason the measuring must be carried out with a high degree of precision in order to be able to calculate where the hole will be drilled.
Gearboxes can be used in order to achieve rotations in the joints of the drill booms, whereby it is possible to choose between using expensive special gearboxes without play or cheaper standard gearboxes with play.
U.S. Pat. No. 5,690,184 A shows a rock drilling apparatus, EP 0 523 252 A1 shows suppression of vibrations in a robot arm, EP 1 178 588 A2 shows a stepping motor, and GB 1 304 449 A shows an apparatus for providing coating for a flexible web of material.
A problem when using a gearbox with play, for positioning in a joint in a drill boom is that the force from the inertia in the drill boom can cause the load from the drill boom acting upon the output shaft of the gearbox to shift between the gear-tooth flanks in the gearbox. This “chatter” or oscillation between the gear-tooth flanks arises in particular with low loads on the gearbox, that is when positioning the drill boom before drilling is commenced. Thus, in applications that require great precision, it has been necessary to use gearboxes that do not have any play, which has resulted in the use of expensive gearboxes, as standard gearboxes for transmissions have play of the order of 0.3 degrees which can not be accepted for, for example, positioning.